If you're up to it, you can pull it out and disassemble it to see what you might be able to fix. Could be as simple as a broken wire. The link below is a write-up for the rear duovalve in a S600, but some of it may apply or help.

http://v12uberalles.com/Rear_AC_System.htm

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1995 S600, 1 of 618 (sold)

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